Method and apparatus for seismically retrofitting a household chimney

Information

  • Patent Grant
  • 6199345
  • Patent Number
    6,199,345
  • Date Filed
    Wednesday, September 27, 1995
    28 years ago
  • Date Issued
    Tuesday, March 13, 2001
    23 years ago
  • Inventors
  • Examiners
    • Chilcot; Richard
    Agents
    • Alford; William E.
Abstract
A method and apparatus for seismically retrofitting pre-existing household brick chimneys is described to meet current building codes. Vertical steel rebar and horizontal chimney anchors are inserted into the chimney and a polymer concrete is poured between the liner and the brick masonry thereby bonding to the chimney, the vertical steel rebar, and the horizontal chimney anchors. The horizontal chimney anchors are coupled at one end to the external face of the chimney and at another end to the interior frame of the house. Holes are vertically drilled into the masonry of the chimney base in order to anchor the steel rebar at one end. Holes are drilled through the chimney and into the interior of the house at an appropriate point to properly anchor the pre-existing chimney to the house. A polymer concrete that has similar heat properties to other fireplace components is poured into the chimney around a chimney liner in order to reinforce and provide resiliency.
Description




FIELD OF THE INVENTION




This invention relates generally to a chimney. More particularly, the invention relates to seismic upgrades of pre-existing household chimneys in order to meet earthquake standards.




BACKGROUND OF THE INVENTION




In older existing homes built in areas prone to earthquakes, brick chimneys may crack or be completely destroyed during an earthquake. If a brick chimney is significantly damaged during an earthquake it may cause bricks to fall to the ground possibly injuring persons or property nearby. To avoid this in certain cities and counties, earthquake standards for new chimney construction have been introduced. The Uniform Building Code sets minimum standards while cities may impose higher standards for new chimney construction. Furthermore certain cities have required that preexisting chimneys be brought up to code when any work is to be performed upon the masonry unit of a chimney or fireplace. Also, real estate sales contracts were written which required a seller to repair the fire, safety, and structural defects of older pre-existing chimneys upon the transfer of title to a home. Previously to meet building codes it required the removal of all or portions of the old chimney and then construction of a new chimney that would meet the building codes.




In older homes and geographic areas without earthquakes, chimneys may be non-reinforced.

FIG. 1A

represents a cutaway view of a non-reinforced chimney


112


A resting upon a fireplace


110


. The outer facade of the fireplace


110


and chimney


112


, also referred to as the brick masonry unit


128


, is made up of brick


160


and mortar


162


. Within chimney


112


A there is an airspace


103


and a chimney liner


104


. Many older homes may not have the chimney liner


104


and rely on the brick masonry unit


128


to direct heat and smoke up through the chimney and out into the atmosphere.




Chimneys may be designed into a home in three broad categorical groupings. One group of chimneys may be designed and constructed such that the fireplace and chimney are external to a home and the back face of the chimney adjoins a homes wall for a significant portion of the chimney height. This first group will be referred to as “external wall chimney”. A second group of chimneys may designed and constructed such that the fireplace and chimney are external to a home and the chimney is freestanding above the roof-line for a significant portion of the chimney height. This second group will be referred to as “external roof chimney”. A third group of chimneys may be designed and constructed such that the fireplace is completely internal to the home and the chimney only rises above the roof for a short portion of the chimney. This third group of chimneys will be referred to as “internal chimneys”. The construction codes for a given city will vary depending upon the above type of chimney that is designed and constructed in a home. For example, pre-existing internal chimneys require no horizontal reinforcement or anchorage because there is little danger that an internal chimney will fall on persons or property.




As earthquakes caused damage to chimneys in geographic areas prone to earthquakes, real estate sales contracts were written which required a seller to repair the fire, safety, and structural defects of older pre-existing chimneys upon the transfer of title to a home. Initially only a horizontal tie or anchorage at the roof-line or plating was installed.

FIG. 1B

represents early attempts to satisfy the horizontal reinforcement requirements for pre-existing external wall chimneys that were flush to a house wall. In

FIG. 1B

, chimney


112


B has an external strap


120


attached to an external wall


125


by use of bolts


121


. This is similar to a brace illustrated in U.S. Pat. No. Des. 285,411 invented by Nathaniel J. Mahoney having a filing date of Nov. 30, 1983. One disadvantage to the use of the external strap


120


is that chimney


112


B may break or crack just above the external strap


120


. Another disadvantage to the use of the external strap


120


is that the bolts


121


may break free from the wall


125


because of the pulling force exerted by the weight of the chimney is so large that the wall can not bear it. Generally the bolts


121


are inserted into holes in the wall


125


that have been drilled through cement. Another disadvantage to the use of the external strap


120


is that it does not satisfy the Uniform Building Code requirements for horizontal reinforcement and anchorage of preexisting chimneys. It is desirable that an external strap or anchor for a preexisting chimney be bolted to the internal frame of a house and meet current building code requirements.




As stronger earthquakes occurred, it became known that a horizontal anchorage such as illustrated by

FIG. 1B

was not enough to prevent parts of a chimney from falling in the case of the external wall or external roof chimney types. Vertical reinforcement was introduced into the pre-existing chimney.

FIG. 1C

illustrates an early attempt at providing the vertical reinforcement of a preexisting chimney in order to meet these initial codes. Chimney


112


C rests upon a fireplace


110


. A mortar cap


114


completes the top of the chimney


112


C while a spark arrestor


116


is attached to either a chimney liner


104


or the mortar cap


114


.




In order to reinforce the chimney and meet the initial codes, the mortar cap


114


was removed and a hole


106


was drilled into the mortar of the firebox at each corner of the chimney. A mortar cement


105


was placed in each hole


106


and steel rebar


100


of one-half inch or five-eights inch diameter extending the length of the chimney


112


C was inserted into each hole


106


. A mortar cement


102


was poured into the void between the brick chimney and the chimney liner


104


to further solidify the chimney. To reinforce the mortar cap, a bond beam


108


of two wraps of a No. 2 pencil steel rod was placed around the four steel rebars


100


. While this technique met the earlier code for vertical reinforcement of pre-existing chimneys it did little to meet the horizontal reinforcement and anchorage required by newer codes. Also the use of cement as a filler added extra weight to a chimney such that foundations were unable to support it.




The 1991 Uniform Building Code for new home and new chimney construction in earthquake prone zones, such as seismic zone


4


, require a chimney to be affixed to a house's frame in order that a chimney does not fall away from the house during an earthquake.

FIG. 2A

illustrates the use of a mason's anchor strap


220


that is attached to a wooden beam


221


by nut/bolt assemblies


222


and


223


that satisfies the 1991 Uniform Building Code seismic anchorage requirement for new chimney construction. The mason's anchor strap


220


is inserted into the chimney while it is being constructed. The wooden beam


221


is attached to the homes joists and the mason's anchor strap


220


is attached to the wooden beam


221


during the homes construction. Horizontal reinforcement in newly constructed chimneys is provided by horizontal reinforcing ties


208


A-N at eighteen inch intervals using a minimum of one quarter inch diameter steel such as a number two pencil rod that horizontally ties the vertical steel rebars


100


together.





FIG. 2B

illustrates a new mason's anchor strap


220


with the nut/bolt assemblies


222


and


223


. The mason's anchor strap


220


has a flat tongue


234


with two holes


232


and


233


for bolting the strap to either wooden beam


221


or wooden beam


219


. Having two holes


232


and


233


in the mason's anchor strap


220


reduces the strength of the strap and is disadvantageous. The flat steel tongue of the anchor strap


220


is twisted ninety degrees at


235


such that a hooked blade


236


is created. The hooked blade


236


is twisted around the steel rebar


100


such as illustrated in FIG.


2


C and then the tongue


234


is bolted to the wooden beam


221


or


219


.

FIG. 2C

illustrates how two mason's anchor straps


219


and


220


are wrapped around the steel rebar


100


in order to provide for horizontal reinforcement and anchorage of the chimney. As a mason constructs the chimney brick by brick and liner by liner, horizontal reinforcing ties


208


A-N are added at eighteen inch intervals. A mason may also fill the area


205


between the masonry work


128


and the chimney liner


104


with mortar if it is a reasonably small area. A mason continues in this manner in order to construct a new chimney having horizontal and vertical reinforcement.




Use of the mason's anchor strap


220


on preexisting chimneys physically requires that a chimney be at least broken down to the roof line. Installation of the horizontal reinforcing ties


208


A-N at eighteen inch intervals requires that a majority of a pre-existing chimney be dismantled. Other codes in some cities require a pre-existing chimney to be completely broken down to the fireplace and then completely rebuilt in order to insert the mason's strap and horizontal reinforcing ties. After installing the mason's anchor strap


220


in a chimney, the mason must reconstruct the remaining portion of the chimney using new mortar. Dismantling a preexisting chimney and reconstructing a new chimney requires an expensive investment in labor and new materials. It is desirable that a preexisting chimney be retrofitted to meet horizontal reinforcement requirements without partially or completely removing the existing brick chimney thereby reducing the investment in labor and materials. It is desirable that a preexisting brick chimney be anchored to the house without partially or completely removing the existing brick chimney.




BRIEF SUMMARY OF THE INVENTION




A method and apparatus for seismically retrofitting preexisting household brick chimneys is described to meet current building codes. Vertical steel rebar and horizontal chimney anchors are inserted into the chimney and a polymer concrete is poured between the liner and the brick masonry thereby bonding to the chimney, the vertical steel rebar, and the horizontal chimney anchors. The horizontal chimney anchors are coupled at one end to the external face of the chimney and at another end to the interior frame of the house. Holes are vertically drilled into the masonry of the chimney base in order to anchor the steel rebar at one end. Holes are drilled through the chimney and into the interior of the house at an appropriate point to properly anchor the pre-existing chimney to the house. A polymer concrete that has similar heat properties to other fireplace components is poured into the chimney around a chimney liner in order to reinforce and provide resiliency.




It is an object of the invention to retrofit a pre-existing chimney to current earthquake building code standards.




Another object of the invention is to retrofit a pre-existing chimney without removing or dismantelling the brick masonry unit.




A still further object of the invention is to horizontally and vertically reinforce a pre-existing chimney.




A still further object of the invention is to horizontally anchor a pre-existing chimney to the frame of a home.











BRIEF DESCRIPTIONS OF THE DRAWINGS





FIG. 1A

shows a cutaway diagram of a front view of a fireplace with a non-reinforced external wall chimney.





FIG. 1B

shows a front view of a fireplace and an external wall chimney using a prior art chimney strap for horizontal anchorage.





FIG. 1C

shows a cutaway diagram of a front view of a fireplace and an external wall chimney that illustrates the vertical reinforcement used to retrofit older chimneys.





FIG. 2A

shows a cross sectional side view of a fireplace and chimney illustrating the horizontal reinforcement used in the construction of new chimneys.





FIG. 2B

illustrates a top view of a new uninstalled mason strap that is used in the construction new chimneys.





FIG. 2C

is a cross sectional top view of the new chimney construction of

FIG. 2A

illustrating the masons strap as horizontal reinforcement.





FIG. 3A

illustrates a cross sectional side view of a fireplace and external wall chimney showing the horizontal tie of the present invention.





FIG. 3B

illustrates a cross sectional top view of the wallmounted chimney and horizontal tie of the present invention.





FIG. 3C

illustrates a cutaway frontal view of the wallmounted chimney reinforced by the horizontal tie of the present invention.





FIG. 3D

illustrates a magnified side view of the horizontal tie of the present invention.





FIG. 3E

illustrates a magnified rear view of a bracket of the horizontal tie of the present invention that may be coupled to the joist or rafter.





FIG. 4A

illustrates a cross sectional side view of a external roof chimney and fireplace showing a second embodiment of the present invention.





FIG. 4B

illustrates a cross sectional top view of the external roof chimney and horizontal tie of the second embodiment of the present invention.





FIG. 4C

illustrates a magnified side view of the horizontal tie of a second embodiment of the present invention.





FIG. 5A

illustrates the cross sectional top dimensions of a wallmounted chimney in order to compute the strength of materials for the present invention.





FIG. 5B

illustrates the cross sectional side dimensions of a fireplace and external wall chimney in order to compute the strength of materials for the present invention.











DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT




The method and apparatus for providing a seismic retrofit for preexisting household external wall chimneys is illustrated by

FIGS. 3A-3E

. The seismic retrofit for preexisting household external roof chimneys is illustrated by

FIGS. 4A-4C

. For either pre-existing chimney type the chimney is horizontally and vertically reinforced to meet current construction codes in order to withstand a reasonable size earthquake without harming persons or property.




External Wall Chimney





FIG. 3A

illustrates fireplace


110


and an external wall chimney


112


E having the horizontal and vertical reinforcement of the present invention. Chimney


112


E has a majority of its back side adjacent to wall


320


. The horizontal reinforcement and anchorage provided by the present invention, referred to as a “chimney anchor”, is illustrated at


300


. Fireplace


110


includes the damper


230


, lintel


224


, and firebrick


226


. Chimney


112


E is coupled to fireplace


110


at its base and the mortar cap


114


and spark arrestor


116


at its top. The mortar and brick to construct the chimney


112


E and fireplace


110


is collectively referred to as the brick masonry unit


128


. Chimney


112


E may further include a chimney liner


104


that may or may not have been present prior to the installation and method of the present invention. The non-reinforced chimney depicted by

FIG. 1A

may have not included the chimney liner


104


.




The vertical reinforcement for chimney


112


E includes at least four steel rebar


100


inserted within drilled holes


106


within the mortar


163


at the base of the chimney. The steel rebar


100


is a #4 (one half inch diameter) or #5 (five-eights inch diameter) steel rebar and is held within the holes by epoxy


305


. To further horizontally and vertically reinforce the chimney and bond the steel rebar


100


and the “chimney anchor”


300


to the chimney, a reinforcement compound such as a polymer concrete


302


may be used to fill in the space between the chimney liner


104


and the brick masonry unit


128


. The polymer concrete is preferable because of its resilience and its heat withstanding capabilities and properties that are similar to a terrocata chimney liner and the firebrick


226


. Generally the polymer concrete is non-flammable and has a similar coefficient of heat expansion as that of a terrocata chimney liner and firebrick. To make polymer concrete a bonding agent such as “QUICKRETE Concrete Bonding Adhesive manufactured by the Quickrete Company of Atlanta, Ga. or “CEMLOK NE” bonding agent manufactured by the Conrad Sovig Co. Inc. of San Francisco, Calif. is added and mixed into concrete mortar. Because of its heat properties the polymer concrete can fill in voids in either the chimney liner


104


or the brick masonry unit of the chimney. Other reinforcing compounds including polymer concrete are further described in the book entitled “Fiber-Reinforced Cement Composites” by Perumalsamy N. Balaguru and Surendra P. Shah published by Mcgraw-Hill, Inc. If a chimney is large and requires an excess amount of polymer concrete the chimney may become too heavy in which case either a lightweight concrete aggregate may substituted such as Portcosta which is distributed by Granite Rock of San Francisco or a method of generating a reinforced web of polymer concrete can be used. Other lightweight concrete aggregates are further described in the book entitled “Lightweight Concrete” by Andrew Short and William Kinniburgh published by Wiley & Sons, Inc. To provide further vertical reinforcement, the bond beam


108


, being a #2 steel rod is wrapped around the steel rebar


100


in the mortar cap


114


.




The horizontal reinforcement of chimney


112


E is further provided by two “chimney anchors”


300


and


301


one of which is depicted by the cross-sectional side view of FIG.


3


A. In

FIG. 3A

“chimney anchor” is bolted against the exterior face of the chimney


112


E and attached to the frame of the house at joist


322


by the bracket


316


.

FIGS. 3B-3E

better illustrate the installation of the two chimney anchors.





FIG. 3B

illustrates a cross-sectional top view of chimney


112


E having the two chimney anchors


300


and


301


.

FIG. 3D

illustrates a magnified side view of each chimney anchor which consist of a threaded steel shaft


314


, malleable washer


312


, nut


310


, bracket


316


, bolt assemblies


319


A and


319


B, steel washer


313


, and a nut


311


. Referring to

FIG. 3B

, the threaded steel shaft


314


is approximately ten feet in length and is either a one half inch or five-eights inch diameter and is inserted into holes drilled through the chimney


112


E and wall


320


. As illustrated by

FIG. 3B

, holes are drilled through the chimney


112


E in which to insert the shaft


314


between the steel rebar


100


and the left and right sides of the chimney, through the front and rear sides of the chimney, and may be drilled through the brick


160


or preferably through the mortar


162


as illustrated by FIG.


3


C. Holes are further drilled through the wall


320


at the roof-line


321


. The threaded steel shaft interior to the house may be shortened if necessary by sawing off a portion of the threaded steel shaft. At the external end of the shaft


314


a malleable washer


312


three inches in diameter with a one inch or seven-eights inch diameter hole is held against the chimney


112


E by a nut


310


. Referring to

FIG. 3D

bracket


316


is inserted over the steel shaft through hole


315


. Bracket


316


may be a “Simpson STRONG TIE” bracket model number HD2A; NER-393 and is described in U.S. Pat. No. 4,665,672. Bracket


316


is then bolted to either a joist or a rafter


322


of the house by use of nut/bolt assemblies


319


A and


319


B.

FIG. 3E

illustrates nut/bolt assembly


319


A in detail and how bracket


316


is held against the joist or rafter


322


. A bolt


318


with a washer


332


is inserted through a hole in bracket


316


and a hole drilled through the joist or rafter


322


. A washer


330


is inserted over the bolt


318


and a nut


317


is threaded onto the bolt and tightened such that the bracket is held tightly against the joist or rafter


322


of the house. The diameter of bolt


318


, washer


332


, washer


330


, and nut


317


depends upon the force applied by the weight of the chimney. Typically the diameter may be either one-half inch or five-eights inch. Referring to

FIG. 3D

, steel washer


313


is then inserted over the threaded steel shaft


314


and nut


311


is tightened in order to create tension in the steel shaft


314


and hold the chimney


112


E to the house.




External Roof Chimney





FIG. 4A

illustrates fireplace


110


and an external wall chimney


112


F having horizontal and vertical reinforcement of a second embodiment of the present invention. Chimney


112


F has a majority of its chimney height above the roof-line. The horizontal reinforcement provided by the second embodiment of the present invention, also referred to as “chimney anchor”, is illustrated at


400


. The fireplace


110


is identical to that of FIG.


3


A and is described above. Chimney


112


F is similar to chimney


112


E described above except for the fact that a majority of the chimney rises above the roof-line due to a semi-vaulted ceiling and the apparatus


400


used for horizontal reinforcement and anchorage is slightly different.




The vertical reinforcement for chimney


112


F is similar to the vertical reinforcement for chimney


112


E that is described above. The reinforcement compounds for chimney


112


F are similar to the reinforcement compounds used for chimney


112


E.




The horizontal reinforcement and anchorage of chimney


112


F is further provided by two “chimney anchors”


400


and


401


one of which is depicted by the cross-sectional side view of FIG.


4


B. In

FIG. 4A

, the “chimney anchor”


400


is bolted against the exterior face of the chimney


112


F and attached to the frame of the house at rafter or joist


422


by the bracket


316


.

FIGS. 4B-4C

better illustrate the installation of the two chimney anchors


400


and


401


.





FIG. 4B

illustrates a cross-sectional top view of chimney


112


F having the two chimney anchors


400


and


401


.

FIG. 4C

illustrates a magnified side view of each chimney anchor which consist of a threaded steel shaft


314


, malleable washers


312


and


412


, nuts


310


and


410


, bracket


316


, bolt assemblies


319


A and


319


B, steel washers


313


,


313


B, and nuts


311


,


311


B. Referring to

FIG. 4B

, the threaded steel shaft


314


is approximately ten feet in length and is either a one half inch or five-eights inch diameter and is inserted into holes drilled through the chimney


112


F and roof


420


. As illustrated by

FIGS. 4A-4C

, holes are drilled through the chimney


112


F in which to insert the shaft


314


. The holes for chimney anchor


400


is drilled between the steel rebar


100


and the left side of the chimney, through the front and rear sides of the chimney, and may be drilled through the brick


160


or preferably through the mortar


162


. The holes for chimney anchor


401


is drilled between the steel rebar


100


and the right side of the chimney, through the front and rear sides of the chimney, and may be drilled through the brick


160


or preferably through the mortar


162


. At the external end of the shaft


314


, the malleable washer


312


three inches in diameter with a one inch or seven-eights inch diameter hole is held against the front side of chimney


112


F by the nut


310


. Additionally, the malleable washer


412


three inches in diameter with a one inch or seven-eights inch diameter hole is held against the rear side of chimney


112


F by the nut


410


. Nut


410


is tightened such that tension is created along the steel shaft


314


between nuts


410


and


310


. The threaded steel shaft interior to the house that is inserted through the roof may be shortened if necessary by sawing off a portion of the threaded steel shaft


314


.




Referring to

FIG. 4C

, nut


311


B is threaded onto the steel shaft


314


and then washer


313


B is slid onto the threaded steel shaft as well. Next, bracket


316


is inserted over the steel shaft through hole


315


. Bracket


316


may be a “Simpson STRONG TIE” bracket model number HD2A; NER-393 and is described in U.S. Pat. No. 4,665,672. Bracket


316


is then bolted to either a rafter


422


or a joist of the house by use of nut/bolt assemblies


319


A and


319


B.

FIG. 3E

, described above, illustrates the nut/bolt assembly


319


A in detail and how bracket


316


is similarly held against the rafter or joist


422


. Referring to

FIG. 4C

, steel washer


313


is then inserted over the threaded steel shaft


314


and nut


311


is then threaded onto the steel shaft. Nuts


311


and


311


B are tightened against the bracket


316


in order to create tension in the steel shaft


314


and hold the chimney


112


F to the house.




Method of Seismically Retrofitting Pre-existing Chimneys




Prior to retrofitting a pre-existing chimney it must be inspected to be sure that the non-reinforced chimney


112


A such as illustrated by chimney


112


A in

FIG. 1A

is sound and can be reinforced. If the non-reinforced chimney is so badly cracked or damaged the chimney must be completely rebuilt. The pre-existing chimney is visually inspected for cracks and is also pull tested by a human to see if there is significant lateral movement or rotation in the chimney that would prevent adding the horizontal and vertical reinforcement of the present invention. The spark arrestor


116


, if one is in place, is removed to determine if the chimney


112


A has a chimney liner


104


and in order to make a computation of space


103


based on the dimensions of the chimney. Further the mortar cap


114


is inspected and may be removed to further determine if it appears there is a space


103


in which drilling equipment can be used to drill the holes


106


in which the steel rebar


100


can be inserted. If there are obstructions that would prevent drilling holes


106


then the chimney may not be capable of retrofitting. If there is no chimney liner


104


a determination may be made as to whether a chimney liner may be installed within the center of the existing brick masonry unit. Nearly all existing chimney building codes require that a chimney liner be installed within the brick masonry unit. If a chimney liner


104


is to be installed then holes


106


may not necessarily need drilling and the steel rebar


100


is cemented into the area around the fireplace along with the first section of the chimney liner


104


.




Once it is determined that the chimney can be retrofitted with the horizontal and vertical reinforcement of the present invention, measurements are made of the chimney and fireplace in order to determine if the foundation of the fireplace is strong enough to support the added weight and the strength and positions of the needed materials to provide the horizontal and vertical reinforcement of the present invention. Referring to

FIG. 5A

, the width (WC)


501


of the chimney and length (LC)


502


of the chimney are measured. The width (WL)


503


of the chimney liner and the length (LL)


504


of the chimney liner are measured. From these measurements the amount of area (A) between the chimney liner and the mortar unit can be approximated by the equation:








A


=[(


WC×LC


)−(


WL×LL


)].  Eq. 1






Referring to

FIG. 5B

, the height of the fireplace (HF)


510


and the total height of the chimney (HC)


512


is measured. An inspection of the attic of the home is made and a determination is made as to where the two chimney anchors may be installed. Knowing where the chimney anchors will be installed the distance between the top of the chimney and the chimney anchor (H


1


)


521


can be determined. From these measurements the height of the chimney anchor(HB)


522


that provides bracing can be determined by the equation:








HB=HC+HF−H




1


  Eq. 2






The total height (HT) of the structure at the top of the chimney can be determined from the equation:








HT=HC+HF


  Eq. 3






Knowing these measurements, a professional engineer can make estimates as to the chimney weight, the vertical bending force upon the steel rebar


100


, and the horizontal force upon the chimney anchor


300


or


400


can be computed to assure the proper diameter of bolts, steel shaft and rebar has been selected. Consider for example a chimney having the following measurements:




H


1


=5 ft, HC=11 ft, HF=7 ft,




WC=17 in, LC=30 in, WL=8 in, LL=17 in.




The area (A) between the chimney liner and the mortar unit is computed by the Eq. 1:








A


=[(17 in×30 in)−(8 in×17 in)]=374 in


2


.






The height of the chimney anchor is computed by Eq. 2 to be:








HB=


11 ft+7 ft−5 ft=13 ft.






The total height of the structure is computed by Eq. 3 to be:








HT=


11 ft+7 ft=18 ft.






Next the chimney weight per foot of height is computed by the equation:








W=A


×[(120 lbs/ft


3


)/(144 in


2


/ft


2


)]  Eq. 4






The number (120 lbs/ft


3


) represents the weight of masonry per cubic foot while the number (144 in


2


/ft


2


) is a conversion factor. Using the example values the chimney weight per foot computes to be:








W=


374 in


2


×[(120 lbs/ft


3


)/(144 in


2


/ft


2


)]=312 lbs/ft






Next the horizontal force applied by an earthquake to the chimney is computed with reference to the top of the chimney by the equation provided in the 1991 Uniform Building Code:








F




p


=ZIC


p


W  Eq. 5






The factor Z is a seismic zone factor and varies depending upon the geographic location of the chimney. The highest zone is zone


4


such that the Z factor becomes 0.40. The factor I is an occupancy factor in which case for a standard occupancy structure such as a home the value is 1.00 for earthquake computations. The factor C


p


is a horizontal force factor and its value varies depending upon the height measurements above. If the chimney anchor is fastened below the roof such as in FIG.


3


A and the chimney extends above the chimney anchor less than one half the total height of the structure, then C


p


=0.75. If the chimney anchor is fastened above the roof such as in

FIG. 4A

at or above the center of mass or if the chimney extends above the chimney anchor more than one half the total height of the structure when the chimney anchor is fastened below the roof such as in

FIG. 3A

, then C


p


=2.00. Continuing with the example as illustrated by

FIG. 5B

, we need to know if:








H




1


<(


HT/


2)  Eq. 6






to determine the value for C


p


. Substituting in the values from above we find:






5 ft<(18 ft/2) or 5 ft<9 ft






such that the value for C


p


is 0.75. Now knowing all variables for Eq. 5 we can compute the horizontal force per foot to be:








F




p


=(040)(1.00)(0.75)(312 lbs/ft)=93.6 lbs/ft.






Now knowing how much force will be applied by the earthquake, bending of the chimney occurring at the chimney anchor can be computed in order to determine the diameter of the steel rebar


100


necessary to avoid a tension failure and toppling of the chimney above the chimney anchor. The area of the steel (As) required to resist tension forces may be computed from:








As=M




1


/(


ad


)  Eq. 7.






In determining the bending of the chimney occurring at the chimney anchor, “M


1


” is the moment applied to the chimney by the earthquake force, “d” is the distance from the compression face of the brick to the center of the reinforcing steel that is being stretched, and “a” is equal to the (average j-value)*(f


s


/12,000). f


s


is the allowed stress of steel in steel reinforced concrete and the average j-value approximates the center of gravity where there is a couple that resists a moment. Knowing the compressed stress for the applied concrete (f′c) and the allowed stress of steel (f


s


), a table for Rectangular Sections of steel reinforced concrete that approximates the reinforced chimney may be used to determine the value for “a”. For example consider f′c is 2000 and f


s


is 20,000 lbs/in


2


, using Table 1 for rectangular sections from the Reinforced Concrete Design Handbook by Thor Germundsson et al, the value for “a” is 1.44. The value for “d” is illustrated by “d”


506


in FIG.


5


A. For a brick that is four inches in width, the value of “d” may be approximately ten (10 in) inches.




The moment “M


1


” applied to the chimney against two of the steel rebar


100


B may be computed from:








M




1


=(


F




p




*H




1


)*(


H




1


/2)  Eq. 8






From this equation it can be seen that it is preferable to place the chimney anchor such that H


1


is smaller while maintaining a strong attachment to the homes frame. Continuing with the example, moment M


1


may be computed as








M




1


=(93.6 lbs/ft)*5 ft)*(5 ft/2)=1,170 ft-lbs






which can be converted to ft-kips using the conversion factor 1 kip/1000 lbs such that








M




1


=(1,170 ft-lbs)*(1 kip/1000lbs)=1,170 ft-kips.






The area of steel required may now be determined by using Eq. 7.




As=(1.170 ft-kips)/[(1.44 ft-kips/in


3


)*(10 in)]=0.08 in


2


. As discussed above the steel rebar


100


may be a #4 (one half inch diameter, one quarter inch radius) or #5 (five-eights inch diameter, five-sixteenths inch radius) steel rebar. The area of a single steel rebar may be computed from the area equation for a circle:








Ac=πr




2


  Eq. 9






For #4 steel rebar the area may be computed as








Ac


4=(3.14)(0.25 in)


2


=0.20 in


2


.






For #5 steel rebar the area may be computed as







Ac


5=(3.14)(0.3125 in)


2


=0.31 in


2


.




Using the #4 steel rebar as steel rebar


100


B or steel rebar


100


F at each corner of the chimney as depicted in

FIG. 5A

, the area of steel for the stretch of back steel rebar


100


B or the front steel rebar


100


F may be computed by the equation








As


4=2*(0.20 in


2


)=0.40 in


2








This value of As4 surpasses the required steel area of 0.08 in


2


computed above. If this was not the case a larger diameter steel rebar would have been chosen.




Next the lateral force at the eave


530


is determined in order to compute the bolts and threaded steel shaft to be used in assembling each chimney anchor. Taking moments at the top of the fireplace the force at the eave may be calculated using the equation:








Fe


=[(


F




p


)*(


HC


)


2


]/[2*(


H




2


)]  Eq. 10






Using the values from above we may compute the force from








Fe


=[(93.6 lbs/ft)*(11 ft)


2


]/[2*(6 ft)]=943.8 lbs.






The force upon each chimney anchor is one half the total force Fe or 471.9 lbs which is used to compute the diameter of the bolts


318


, the threaded steel shaft


314


and the strength of the malleable washer


312


.




The tension force capability of the threaded steel shaft


314


may be computed from the equation:








Ft


=(


At


)(


SK


)(


SI


)  Eq. 11.






“At” is the tension area of the threaded steel shaft


314


, “SK” is the allowable stress in the threaded steel shaft


314


, and “SI” is an earthquake increase factor due to the limited duration of the load. For a one-half inch diameter threaded steel shaft


314


the values of these factors from the 1991 Uniform Building code are:








At=


0.14 in


2




, SK=


20,000 lbs/in


2




, SI=


1.33.






The tension capability of the threaded steel shaft


314


may be computed to be








Ft


=(0.14 in


2


)(20,000 lbs/in


2


)(1.33)=3724 lbs.






Comparing Fe with Ft, it is determined if the tension capability of the threaded steel shaft


314


exceeds the force applied at the eave such that it is a proper diameter. In the example above, Ft (3724 lbs) is greater than Fe (943.8 lbs) such that the diameter of the steel shaft


314


is sufficient. If Ft was less than Fe, then a larger diameter threaded steel shaft


314


would be selected such as five-eights of an inch.




The shear force capability of the bolts


318


holding one bracket


316


to the rafters or joist


322


may be computed using the equation








Fs


=(


B


)(


F


)(


SI


)  Eq. 12






“B” is the number of bolts


318


used to hold bracket


316


, which in this case is two in number. “F” is the allowable shear stress on the bolts


318


obtained from Table 8.2B of the 1991 Edition of the “National Design Specification for Wood Construction”. Considering the bolts


318


to be one-half inch in diameter and the rafter or joist


322


to be one and one-half inch in thickness, the value for F is 530 lbs/bolt. “SI” is the earthquake increase factor due to the limited duration of the load. From the 1991 Uniform Building code, SI=1.33. The shear force may then be computed to be








Fs


=(2 bolts)(530 lbs/bolt)(1.33)=1409 lbs






Comparing Fs with Ft, it is determined if the shear force capability of the bolts


318


exceed the force applied at the eave such that it is a proper diameter. In the example above, Fs (1409 lbs) is greater than Fe (943.8 lbs) such that the diameter of the bolts


318


is sufficient. If Fs was less than Fe, then a larger diameter bolt would be selected such as five-eights of an inch.




Through the use of the above example and equations, the strength of materials for external roof chimneys as illustrated by

FIGS. 4A-C

may be determined as well.




After these calculations are made these are submitted to the city building department for approval. Once approved and a permit is issued the retrofit of the pre-existing chimney may begin.




Referring to

FIG. 1A

, the spark arrester


116


and the mortar cap


114


are removed if not already. Drilling equipment is first used to drill the two horizontal holes through the front and rear of the chimney


112


E or


112


F and through the wall


320


or roof


420


as required for the two chimney anchors


300


and


301


or


400


and


401


. It is preferable to drill through a mortar joint of the mortar


162


of the brick masonry unit


128


instead of the brick


160


because of the ease and speed of drilling the horizontal holes. The holes for the chimney anchors in the chimney are drilled on each side of the chimney liner


104


in the space


103


between the chimney liner and the brick masonry unit


128


such that each chimney anchor will be inserted outside the steel rebar


100


. Next the drilling equipment is used to drill through obstructions over the height of the chimney in the space


103


and into at least four holes


106


near the corners of the chimney box mortar


163


as illustrated by FIG.


1


C. Drilling into the chimney box mortar


163


assures that the steel rebar


100


extends the full height of the chimney. The rubble and debris generated by the drilling is vacuumed out to clear holes


106


by using vacuuming equipment.




Steel rebar


100


is then inserted into each of the holes


106


and an epoxy


305


is used to fill the holes


106


and hold one end of each of the steel rebar


100


. Next the two chimney anchors


300


and


301


or


400


and


401


as the case may be are installed into the horizontal holes drilled into the chimney and the house.




In the case of chimney anchors


300


and


301


as illustrated by

FIGS. 3A-3E

they are installed as follows. Referring to

FIG. 3B

, the malleable washer


312


is inserted over an external end of the threaded steel shaft


314


. The nut


310


is screwed onto the threaded steel shaft


314


such that a few threads are left exposed on the external end. The internal end of the steel shaft


314


is inserted into holes drilled through the chimney


112


E. The internal end is pushed through the holes drilled into the chimney and the wall


320


. A hammer is used on the external end to finally pound the steel shaft


314


in place as well as damage a few exposed threads such that the malleable washer


312


and nut


310


will not loosen and fall off the steel shaft. The interior end of the threaded steel shaft may be shortened if necessary by sawing off a portion of the threaded steel shaft. However, it is preferable to use the entire length in order to grab the frame of the house more closely to the center of mass of the house. Referring to

FIG. 3D

, bracket


316


is inserted over the internal end of the steel shaft through hole


315


. Bracket


316


is then bolted to the either a joist or a rafter


322


of the house by use of nut/bolt assemblies


319


A and


319


B. It is preferable that a joist be used to mount the bracket. Holes where the bracket


316


may be mounted are drilled through the wooden joist or rafter


322


.

FIG. 3E

illustrates nut/bolt assembly


319


A in detail and how bracket


316


is held against the joist or rafter


322


. A bolt


318


with a washer


332


is inserted through a hole in bracket


316


and the hole drilled through the joist or rafter


322


. A washer


330


is inserted over the bolt


318


on the opposite side of the joist or rafter


322


and a nut


317


is threaded onto the bolt and tightened such that the bracket is held tightly against the joist or rafter


322


of the house. Referring to

FIG. 3D

, steel washer


313


is then inserted over the threaded steel shaft


314


and nut


311


is tightened in order to create tension in the steel shaft


314


and hold the chimney


112


E to the house.




In the case of chimney anchors


400


and


401


as illustrated by

FIGS. 4A-4C

they are installed as follows. Referring to

FIG. 4B

, the malleable washer


312


is inserted over an external end of the threaded steel shaft


314


. The nut


310


is screwed onto the threaded steel shaft


314


such that a few threads are left exposed on the external end. The internal end of the steel shaft


314


is inserted into holes drilled through the chimney


112


E. The internal end is pushed through the holes drilled into the chimney until the internal end of the threaded steel shaft


314


is exposed on the back side of the chimney. Malleable washer


412


is inserted over the internal end of the steel shaft and the nut


410


is threaded onto the internal end. If the threads on the internal end are damaged by being pushed through the chimney they are cleaned up by using a die of the appropriate size such that nut


410


can be threaded onto the steel shaft


314


. The steel shaft is then further pushed through the chimney and inserted through the holes drilled into the roof


420


. As the steel shaft is pushed through, the nut


410


is further tightened onto the steel shaft towards the chimney


112


F carrying the malleable washer


412


with it. A hammer is used on the external end to finally pound the steel shaft


314


in place as well as damage a few exposed threads such that the malleable washer


312


and nut


310


will not loosen and fall off the steel shaft. Nut


410


is tightened such that tension is created along the steel shaft


314


between nuts


410


and


310


. The threaded steel shaft interior to the house that is inserted through the roof may be shortened if necessary by sawing off a portion of the threaded steel shaft


314


. Referring to

FIG. 4C

, nut


311


B is threaded onto the steel shaft


314


and then washer


313


B is slid onto the threaded steel shaft as well. Next, bracket


316


is inserted over the steel shaft through hole


315


. Bracket


316


is then bolted to either a rafter or joist


422


of the house by use of nut/bolt assemblies


319


A and


319


B.

FIG. 3E

, described above, illustrates the nut/bolt assembly


319


A in detail and how bracket


316


is similarly held against the rafter or joist


422


. Referring to

FIG. 4C

, steel washer


313


is then inserted over the threaded steel shaft


314


and nut


311


is then threaded onto the steel shaft. Nuts


311


and


311


B are tightened against the bracket


316


in order to create tension in the steel shaft


314


and hold the chimney


112


F to the house.




After the steel rebar


100


and the two chimney anchors are installed, another inspection is made by the city building commissioner of the installation. Measurements are actually taken to be sure that the installation of the horizontal and vertical reinforcement are made according to the engineering specifications.




After receiving approval of the installation, a reinforcing compound such as polymer concrete


302


is then poured into the space


103


. If the polymer concrete


302


oozes out through the joints of the brick it is sponged off the brick masonry unit. If it oozes out through the joints or cracks of the chimney liner


104


then it is wiped smooth to the surface of the chimney liner. The polymer concrete


302


is allowed time to cure and then the bond beam


108


is installed around the steel rebar


100


. Mortar cement is then used to pour the mortar cap


114


and it is allowed to cure. Next the spark arrestor


116


is installed to complete the assembly of the retrofitted chimney


112


E or


112


F. Flashing may be added around the holes in the roof


420


in the case of the exterior roof chimneys. The exterior nuts


310


,


410


and malleable washers


312


,


412


may be painted with a rust proof paint such as Rustolium for aesthetic reasons.




While a preferred embodiment of the present invention has been disclosed and described in detail herein, it will be obvious to those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope thereof.



Claims
  • 1. A method for seismically reinforcing a pre-existing brick chimney, the steps comprising:(a) drilling two horizontal holes extending through a width of said pre-existing brick chimney; (b) inserting vertical steel rebar extending approximately the length of said pre-existing brick chimney; (c) inserting a threaded shaft, having a first end and a second end with a washer coupled to said first end of said threaded shaft, into one of said two horizontal holes extending through the width of said pre-existing brick chimney; (d) coupling a bracket to said second end of said threaded shaft; (e) coupling said bracket to either a house joist or a house rafter; and (f) coupling a nut to said second end of said threaded shaft to create tension along said threaded shaft between said pre-existing brick chimney and said house joist or house rafter.
  • 2. The method of claim 1 for seismically reinforcing a pre-existing brick chimney, the steps further comprising:repeating steps (c) through (f).
  • 3. The method of claim 1 for seismically reinforcing a pre-existing brick chimney, the steps further comprising:inserting a chimney liner into the center portion of said pre-existing brick chimney if none is present.
  • 4. The method of claim 2 for seismically reinforcing a pre-existing brick chimney, the steps further comprising:pouring a reinforcing compound between said pre-existing brick chimney and said chimney liner to couple said pre-existing brick chimney, said chimney liner, said threaded shaft, and said vertical reinforcement shafts and to reinforce said pre-existing brick chimney.
US Referenced Citations (12)
Number Name Date Kind
D. 285045 Mahoney Aug 1986
D. 285411 Mahoney Sep 1986
3602468 Stone Aug 1971
4173923 Snook Nov 1979
4190993 Pohlman et al. Mar 1980
4519303 Snook May 1985
4567700 Snook Feb 1986
4686807 Newsome Aug 1987
4731967 Mclauglin Mar 1988
4963191 LaFleur Oct 1990
5291706 Beardsley et al. Mar 1994
5362224 Hasti et al. Nov 1994
Non-Patent Literature Citations (6)
Entry
“1991 Uniform Building Code”, International Conference of Building Officials, pp. 147-148 158-159, 168-169, 184-186, 190, 194, 253, 655-669.
“National Design Specification for Wood Construction”, 1991 Edition, National Forest Products Association, Table 8.2B.
“Reinforced Concrete Design Handbook”, Second Edition, 1955, Table 1.
“Design of Earthquake-Resistant Buildings”, Minoru Wakabayashi, 1986, pp. 178-183.
“Earthquakes, An Architect's Guide to Nonstructural Seismic Hazards”, Henry J. Lagorio, 1990, pp. 148, 165-169, 300.
Ahrens Chimney Technique, Installation Process Brochure, 2 pages, 1992.